KR20210158107A - Method of manufacturing steel material with excellent brittle crack arrest characteristics - Google Patents

Abstract

The present invention provides a method for manufacturing brittle crack arrest steel. According to one embodiment of the present invention, the method for manufacturing brittle crack arrest steel comprises a step of reheating a steel material; a step of hot-rolling the reheated steel material; and a step of cooling the hot-rolled steel material. The step of reheating the steel material includes a step of heating the steel material at a first heating temperature of 925 ℃ to 975 ℃; a step of heating the steel material at a second heating temperature of 1070 ℃ to 1120 ℃; and a step of heating the steel material at a third heating temperature of 1100 ℃ to 1150 ℃.

Description

Method of manufacturing steel material with excellent brittle crack arrest characteristics

The present invention relates to a method for manufacturing a steel material, and more particularly, to a method for manufacturing a steel material having excellent brittle crack stopping properties.

In order to secure the stability of container carriers, the use of Brittle Crack Arrest (BCA) steel has been made mandatory for contract ships since 2014, and maximum strength is required for container ships called hatch coaming. It is applied to the affected area. On the other hand, the recent trend of container ship construction is gradually increasing in size. Recently, 22,000TEU class large container ships are being built. At the same time, the thickness of the brittle fracture stopping property steel material applied to hatch coaming has also increased. It is designed/applied up to the limit of 100 mm. An increase in the thickness of the applied steel increases the weight of the hull, which may adversely affect the transport efficiency by affecting the fuel efficiency and speed reduction of the ship. Accordingly, studies are underway on high-strength steels that maintain brittle fracture stopping characteristics with higher strength than the existing 460 MPa grade brittle fracture stopping characteristics.

As the slab reheating temperature (SRT) increases during the steel manufacturing process, the size of the prior austenite grain size (PAGS) increases, making it difficult to secure brittle crack stopping properties. In addition, by adding precipitation-type alloying elements such as niobium (Nb) and titanium (Ti), there is a phenomenon in which grain growth is suppressed by fine precipitates. ), and in order to effectively apply it, the slab reheating temperature management is important.

Korean Patent Publication No. 2014-0098903

The technical problem to be achieved by the technical idea of the present invention is to provide a method of manufacturing a brittle crack stop characteristic steel.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided a method for manufacturing a brittle crack stopping property steel.

According to an embodiment of the present invention, the method for manufacturing a brittle crack stop characteristic steel, reheating the steel; hot rolling the reheated steel material; And cooling the hot-rolled steel material; including, the step of reheating the steel, heating the steel material at a first heating temperature of 925 ℃ ~ 975 ℃; heating the steel at a second heating temperature of 1070°C to 1120°C; and heating the steel material at a third heating temperature of 1100°C to 1150°C.

According to one embodiment of the present invention, after performing the step of heating at the third heating temperature, the step of re-heating the steel at an extraction temperature of 1050 ° C. to 1100 ° C. may end.

According to an embodiment of the present invention, the step of hot rolling may include: performing primary rolling of the reheated steel in an austenite recrystallization region; and secondary rolling the first rolled steel at a rolling end temperature of 680° C. to 720° C. in an austenite non-recrystallized region.

According to an embodiment of the present invention, in the cooling step, the hot-rolled steel may be cooled to a cooling end temperature of 250° C. to 400° C. at a cooling rate of 4° C./sec to 20° C./sec.

According to an embodiment of the present invention, the steel is, by weight, carbon (C): 0.055% to 0.085%, silicon (Si): 0.10% to 0.20%, manganese (Mn): 1.75% to 1.95%, Phosphorus (P): >0% to 0.012%, Sulfur (S): >0% to 0.002%, Soluble Aluminum (S_Al): 0.015% to 0.050%, Copper (Cu): 0.25% to 0.35%, Niobium (Nb) ): 0.025% to 0.035%, nickel (Ni): 0.65% to 0.75%, molybdenum (Mo): 0.03% to 0.07%, titanium (Ti): 0.008% to 0.018%, and the balance is iron (Fe) and unavoidable It may contain impurities.

According to the technical idea of the present invention, the brittle crack stopping property steel material can have excellent brittle crack stopping properties by suppressing the formation and growth of initial austenite grains through control of reducing the maximum temperature during reheating. In particular, by controlling the maximum temperature in the reheating furnace to 1150° C. or less, the initial austenite grains are finely formed in the steel, and thus, the low-temperature toughness of the surface structure of the steel can be increased. According to this low-temperature toughness increase characteristic, it can be used as a steel material applied to the part supporting the maximum load of the container ship. The steel material reduces the thickness of the applied steel material in response to the recent enlargement of the container ship and can improve the safety of the ship by having the property to stop the propagation of cracks that may occur in the steel material. In addition, by securing the toughness of the surface texture, it is possible to secure the compatibility of small brittle fracture tests such as NRL with large brittle fracture tests.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a flowchart schematically showing a method of manufacturing a brittle crack stopping property steel material according to an embodiment of the present invention.
Figure 2 schematically shows a heat treatment system for performing the step of reheating the steel in the manufacturing method of the brittle crack stop characteristic steel material according to an embodiment of the present invention.
Figure 3 is a graph showing the temperature in the step of reheating the steel in the manufacturing method of the brittle crack stop characteristic steel material according to an embodiment of the present invention.
4 is a scanning electron microscope photograph showing the microstructure of an Example and a Comparative Example of a steel material formed by using the method for manufacturing a brittle crack stopping property steel material according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

1 is a flowchart schematically showing a method of manufacturing a brittle crack stop characteristic steel material according to an embodiment of the present invention.

Referring to Figure 1, the method of manufacturing a brittle crack stop characteristic steel material according to the present invention, reheating the steel material (S10), hot rolling the reheated steel material (S20), cooling the rolled steel material (S30).

Hereinafter, the manufacturing method of the brittle crack stopping characteristic steel of the present invention will be described in detail for each step.

Reheating step (S10)

In general, the steel refining process consists of out-of-furnace refining in which primary refining is performed in a converter, followed by secondary refining by tapping the molten steel of the converter with a ladle. do Usually, deoxidation takes place between primary and secondary refining. The molten steel refined as described above is continuously cast to form a steel material such as a slab.

Reheating the steel material, heating the steel material at a first heating temperature of 925 ℃ ~ 975 ℃; heating the steel at a second heating temperature of 1070°C to 1120°C; and heating the steel at a third heating temperature of 1100° C. to 1150° C.; may include

In addition, after performing the step of heating at the third heating temperature, the step of re-heating the steel at an extraction temperature of 1050 ℃ ~ 1100 ℃ can end.

Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur. The initial slab reheating temperature (SRT) plays a decisive role in PAGS (Prior Austenite Grain size), which affects the size of final grains, and PAGS increases as the initial reheating temperature increases. The reheating temperature can be determined in consideration of solid solution of precipitates, coarsening of austenite grains, and deoxidation.

Figure 2 schematically shows a heat treatment system for performing the step of reheating the steel in the manufacturing method of the brittle crack stop characteristic steel material according to an embodiment of the present invention.

Referring to FIG. 2 , the heat treatment system includes a preheating zone 10 , a first heating zone 20 , a second heating zone 30 , a third heating zone 40 , and a crack zone 50 . The first heating zone 20 , the second heating zone 30 , and the third heating zone 40 may include a heater H. Also, a heater (not shown) may be provided in the preheating table 10 .

The steel M is reheated in the heat treatment system while being transported. Specifically, the steel (M) is preheated to 300 ℃ ~ 400 ℃ in the preheating table (10). Then, the steel (M) is heated to a first heating temperature of 925 ℃ ~ 975 ℃ in the first heating zone 20, heated to a second heating temperature of 1070 ℃ ~ 1120 ℃ in the second heating zone 30, the second 3 It is heated to a third heating temperature of 1100 ℃ ~ 1150 ℃ in the heating zone (40). Then, the steel (M) is cooled in the crack zone 50 is extracted from the heat treatment system at an extraction temperature of 1050 ℃ ~ 1100 ℃.

Hot rolling step (S20)

A step (S20) of hot rolling the reheated steel material is performed. The hot rolling may be performed in two steps as follows.

In the primary rolling step, the reheated steel is primary rolled in the austenite recrystallization region. At this time, the number of passes is 5 to 10 times in order to reach the influence of the rolling force by rolling to the center of the steel, so that the reduction rolling is performed. In addition, by applying low-speed rolling of about 1.2 m/sec to 1.5 m/sec, the reduction ratio per pass is maximized. As the rolling progresses and the thickness of the steel gradually decreases, the rolling reduction of the next pass may be gradually increased so that the rolling force can be transmitted to the center.

In the secondary rolling step, the primary rolled steel is secondary rolled at a rolling termination temperature (FRT) of 680° C. to 720° C. in an austenite non-recrystallized region. When the secondary rolling end temperature is less than 680 ° C., a mixed grain structure may be generated by the abnormal rolling, thereby reducing the physical properties of the steel. In addition, when the secondary rolling end temperature exceeds 720 °C, the strength of the steel and the like may be insufficient.

Cooling step (S30)

A step (S30) of cooling the hot-rolled steel is performed. In the cooling step (S30), the hot-rolled steel is cooled from a cooling start temperature (SCT) of 680°C to 720°C at an average cooling rate of 4°C/sec to 20°C/sec at a cooling end temperature of 250°C to 400°C (FCT) ) to cool.

When the average cooling rate exceeds 20° C./sec, it is advantageous to secure strength, but may cause material deviation in the thickness direction and decrease resistance to brittle fracture.

If the cooling end temperature is less than 250 ℃, there is a problem in that a large amount of low-temperature transformation structure is formed, brittle fracture resistance is lowered. When the cooling end temperature exceeds 400° C., there is a problem in that the strength is insufficient due to the formation of a coarse microstructure.

The steel is, in weight%, carbon (C): 0.055% to 0.085%, silicon (Si): 0.10% to 0.20%, manganese (Mn): 1.75% to 1.95%, phosphorus (P): more than 0% 0.012%, Sulfur (S): >0% to 0.002%, Soluble Aluminum (S_Al): 0.015% to 0.050%, Copper (Cu): 0.25% to 0.35%, Niobium (Nb): 0.025% to 0.035%, Nickel (Ni): 0.65% to 0.75%, molybdenum (Mo): 0.03% to 0.07%, titanium (Ti): 0.008% to 0.018%, and the balance may include iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component included in the brittle crack arresting steel material according to the present invention will be described as follows. In this case, the content of the component elements all mean wt%.

carbon (C)

Carbon may be added in an amount of 0.055% to 0.085% of the total weight of the steel. Carbon is added to ensure the strength of the steel. When the carbon content is less than 0.55%, the strength of the steel may be insufficient, and conversely, when the carbon content exceeds 0.085%, there is a problem in that the low-temperature impact toughness and weldability of the steel material are lowered.

Silicon (Si)

Silicon may be added in 0.10% to 0.20% of the total weight of the steel. Silicon is added as a deoxidizer to remove oxygen in steel in the steelmaking process. In addition, as a solid solution strengthening element, silicon is an element effective in improving the strength of steel and improving ductility. When the content of silicon is less than 0.10%, the above-described effect may be insignificant. When the content of silicon exceeds 0.20%, a large amount of oxide may be formed on the surface of the steel to impair the plating properties of the steel and reduce weldability.

Manganese (Mn)

Manganese may be added in 1.75% to 1.95% of the total weight of the steel. Manganese is an austenite stabilizing element, which increases the strength and toughness of steel and increases the hardenability of steel. When the manganese content is less than 1.75%, it may be difficult to secure strength. When the manganese content exceeds 1.95%, the strength is increased, but segregation may occur, which may cause tissue non-uniformity, and there is a problem of lowering the low-temperature impact toughness. In order to improve the brittle crack stopping properties of steel, it is necessary to improve the toughness of the steel by reducing the content of elements such as carbon and silicon. As a result, it is possible to secure a steel material excellent in brittle crack stopping properties.

Phosphorus (P)

Phosphorus may be added in an amount greater than 0% to 0.012% of the total weight of the steel. Phosphorus partially contributes to the improvement of the strength of steel, but as a representative element that reduces low-temperature impact toughness, the lower the content, the better. However, in order to minimize the phosphorus content to less than 0.012% of the total weight of the steel, the manufacturing cost of the steel increases significantly. In the present invention, the phosphorus content is limited to 0.012% or less of the total weight of the steel.

Sulfur (S)

Sulfur may be added in an amount greater than 0% to 0.002% of the total weight of the steel. Sulfur is an element that is unavoidably contained in the manufacture of steel together with the phosphorus, and forms emulsion-based inclusions (MnS) to reduce low-temperature impact toughness. However, since it is practically difficult to make the sulfur content less than 0.002%, in the present invention, the sulfur content is limited to 0.002% or less of the total weight of the steel.

Soluble Aluminum (S_Al)

Soluble aluminum may be added in 0.015% to 0.050% of the total weight of the steel. Soluble aluminum is an element mainly used as a deoxidizer. It purifies ferrite to improve elongation, and it contributes to stabilizing austenite by increasing carbon concentration in austenite. When the content of soluble aluminum is less than 0.015%, the effect of the addition is insufficient. When the content of soluble aluminum exceeds 0.050%, there is a problem in that the resistance to brittle fracture is lowered.

Copper (Cu)

Copper may be added in 0.25% to 0.35% of the total weight of the steel. Copper is an effective element for increasing strength and improving toughness. When the content of copper is less than 0.25%, the effect of the addition is insufficient. If the content of copper exceeds 0.35%, it may cause surface defects.

Niobium (Nb)

Niobium may be added in an amount of 0.025% to 0.035% of the total weight of the steel. Niobium is one of the elements that greatly affects the strength of steel, and forms carbides or nitrides by combining with carbon and nitrogen. This suppresses grain growth during rolling and refines grains, thereby improving strength and low-temperature toughness. In addition, niobium increases the reheating temperature to increase the rolling reduction in the non-recrystallized region during rolling, thereby improving the strength and impact toughness of steel through grain refinement. When the content of niobium is less than 0.025%, the effect of its addition is insufficient. When the content of niobium exceeds 0.035%, the weldability of the steel is deteriorated, and there is a risk of lowering the CTOD characteristics.

Nickel (Ni)

Nickel may be added in 0.65% to 0.75% of the total weight of the steel. Nickel refines grains and is dissolved in austenite and ferrite to strengthen the matrix. In particular, nickel is an effective element for improving low-temperature toughness. When the content of nickel is less than 0.65%, the effect of the addition is insufficient. When the content of nickel exceeds 0.75%, there may be a problem causing red hot brittleness.

Molybdenum (Mo)

Molybdenum may be added in an amount of 0.03% to 0.07% of the total weight of the steel. Molybdenum is a useful element for improving the strength of steel. When the content of molybdenum is less than 0.03%, the effect of improving the strength is insignificant. When the content of molybdenum exceeds 0.07%, coarse bainite and island martensite (MA) structures may be formed in the center of the thick material to deteriorate the DWTT characteristics. However, since molybdenum is an expensive element and weldability decreases when its content is increased, it is preferable to limit the upper limit to 0.07%.

Titanium (Ti)

Titanium may be added in an amount of 0.008% to 0.018% of the total weight of the steel. Titanium produces Ti(C, N) precipitates with high high-temperature stability, thereby preventing austenite grain growth during welding and refining the structure of the welded portion, thereby improving the toughness and strength of steel. If the content of titanium is less than 0.008%, the effect is insignificant. When the content of titanium exceeds 0.018%, there is a problem in that the low-temperature impact properties of the steel are lowered by generating coarse precipitates, and the manufacturing cost is increased without any further effect of addition.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all details are not specifically mentioned.

Example

Hereinafter, the present invention will be described in detail through Examples, but these are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the scope of the description of these Examples. Content not described here will be omitted because it can be technically inferred sufficiently by a person skilled in the art.

Steel slabs having the compositions shown in Table 1 were prepared. Here, in Examples and Comparative Examples, steel slabs having the same composition were used.

ingredient C Si Mn P S S_Al content(%) 0.062 0.15 1.77 0.0066 0.002 0.027 ingredient Cu Nb Ni Mo Ti content(%) 0.29 0.027 0.70 0.05 0.011

Then, the steel slab was reheated to the reheating temperature in Table 2.

division
reheat temperature first heating temperature
(℃) second heating temperature
(℃) third heating temperature
(℃) extraction temperature
(℃) Example 951 1103 1133 1074 comparative example 983 1106 1212 1089

Figure 3 is a graph showing the temperature in the step of reheating the steel in the manufacturing method of the brittle crack stop characteristic steel material according to an embodiment of the present invention.

Referring to FIG. 3 and Table 2, compared to the Comparative Example, the Example was reheated by setting the heating temperature of the third heating zone to a lower temperature of about 80°C. In addition, the temperature deviation of the first heating zone was ±50°C, the second heating zone was ±70°C, the third heating zone was ±150°C, and the extraction temperature at the cracking zone was ±55°C.

Then, the reheated steel slab was prepared as a steel plate under the process conditions of Table 3.

division rolling method Rolling end temperature
(℃) Cooling start temperature
(℃) Cooling end temperature
(℃) cooling rate
(℃/sec) Example TMCP 720 700 400 4 comparative example TMCP 720 700 400 4

Referring to Tables 2 and 3, Comparative Examples and Examples have different reheating temperature conditions, and in particular, different third heating temperatures. Subsequent hot rolling and cooling were performed under the same conditions. Here, the first heating temperature, the second heating temperature, and the third heating temperature may mean the temperature of the heating region of the heating furnace for reheating the steel.

A tensile test for measuring mechanical properties was performed on the examples of the manufactured steel materials. After the cooling step, the steel material had a yield strength (YP) of 460 MPa to 550 MPa, a tensile strength (TS) of 580 MPa to 660 MPa, and an elongation of 20% to 25%.

In addition, in order to examine the Nil-ductility transition temperature (NDTT) characteristics of the Examples and Comparative Examples, a Naval Research Laboratory (NRL) test was performed at -70°C.

For reference, large brittle fracture tests such as ESSO test or CAT test can be performed to confirm the brittle fracture stopping properties of steel, or small brittle fracture tests such as NRL test can be performed. Recently, a small brittle fracture test that can replace the large brittle fracture test is required for mass production supply. Among various small brittle fracture tests, the NRL test is the one that is predominantly affected by the surface texture and physical properties of steel. In the NRL test, a protrusion is formed on a steel material and an impact load is applied to the opposite surface of the surface on which the protrusion is formed, and whether the protrusion is destroyed or not is observed.

In the case of the comparative example, all ten sets were broken, whereas in the case of the example, all ten sets were not broken. That is, even when the extraction temperature of the steel is almost similar, in the case of the comparative example having a high third heating temperature, fracture occurred. When the steel material is reheated, the surface temperature of the steel material becomes higher than the internal temperature, and accordingly, there is a high possibility that initial grain growth occurs on the surface. have. Since the NRL test is a method of conducting a test on the surface of a steel material, such a difference in grain size may significantly affect the test result.

4 is a scanning electron microscope photograph showing the microstructure of an Example and Comparative Example of a steel material formed by using the method for manufacturing a brittle crack stopping property steel material according to an embodiment of the present invention.

Referring to FIG. 4 , the initial austenite grains (PAGS) before hot rolling, the final microstructure after hot rolling, and the microstructure after the NRL test are shown. In the case of the comparative example, the initial austenite grains had a size of 90 μm or more, whereas in the case of the Examples, the initial austenite grains were 50 μm or less. Therefore, it can be seen that the comparative example has initial austenite grains that are about 1.8 times larger than that of the example. In addition, in the case of the comparative example, the size of the initial austenite grains showed a non-uniform distribution, whereas in the case of the Example, the size of the initial austenite grains were relatively uniformly distributed. It is analyzed that the comparative example is because the growth of initial austenite grains occurs more easily, and large grains are grown while absorbing small grains.

This initial austenite grain distribution causes differences in the final microstructure after hot rolling, and in the case of Comparative Example, many grains elongated in the hot rolling direction were formed, and the size of the grains was also non-uniform. Accordingly, it is analyzed that failure is induced during the NRL test. On the other hand, in the Example, the crystal grains were finely distributed even after hot rolling, and the tensile strength in the rolling direction was not large. By this microstructure, in the case of the example, it is analyzed that destruction is inhibited during the NRL test.

When examining the microstructure on the fracture surface after the NRL test, in the case of the fractured comparative example, irregular structure size distribution and coarse ferrite and low-temperature bainitic ferrite structures were observed. On the other hand, in the case of the unbroken example, a relatively fine tissue distribution was observed.

That is, in the case of the comparative example, as the maximum heating temperature is high during reheating of the steel, the initial austenite grains are increased and the surface structure is coarsened, and the low-temperature phase with low low-temperature toughness is highly likely to be formed after hot rolling. .

The method for manufacturing a steel material with brittle crack stopping properties according to the technical idea of the present invention, by controlling the maximum temperature in the reheating furnace to 1150 ° C. or less, microscopically forms initial austenite grains in the steel, and thus the low temperature of the surface structure of the steel It can increase toughness.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (5)

reheating the steel;
hot rolling the reheated steel material; and
Including; cooling the hot-rolled steel material;
The step of reheating the steel material,
heating the steel at a first heating temperature of 925° C. to 975° C.;
heating the steel at a second heating temperature of 1070°C to 1120°C; and
Including; heating the steel at a third heating temperature of 1100 ° C to 1150 ° C.
A method of manufacturing steel with brittle crack stopping properties. The method of claim 1,
After performing the step of heating at the third heating temperature,
Ending the step of reheating the steel at an extraction temperature of 1050 ℃ ~ 1100 ℃,
A method of manufacturing steel with brittle crack stopping properties. The method of claim 1,
The hot rolling step is,
first rolling the reheated steel in an austenite recrystallization region; and
Secondary rolling of the primary rolled steel at a rolling end temperature of 680 ° C to 720 ° C in an austenite non-recrystallized region; including,
A method of manufacturing steel with brittle crack stopping properties. The method of claim 1,
The cooling step is
Cooling the hot-rolled steel to a cooling end temperature of 250°C to 400°C at a cooling rate of 4°C/sec to 20°C/sec,
A method of manufacturing steel with brittle crack stopping properties. The method of claim 1,
The steel is, in weight%, carbon (C): 0.055% to 0.085%, silicon (Si): 0.10% to 0.20%, manganese (Mn): 1.75% to 1.95%, phosphorus (P): more than 0% 0.012%, Sulfur (S): >0% to 0.002%, Soluble Aluminum (S_Al): 0.015% to 0.050%, Copper (Cu): 0.25% to 0.35%, Niobium (Nb): 0.025% to 0.035%, Nickel (Ni): 0.65% to 0.75%, molybdenum (Mo): 0.03% to 0.07%, titanium (Ti): 0.008% to 0.018%, and the balance contains iron (Fe) and unavoidable impurities;
A method of manufacturing steel with brittle crack stopping properties.

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